The present invention relates, in general, to a new process for the fabrication of submicron, movable mechanical structures, and more particularly to a simplified single-crystal reactive etching process for such structure which is independent of crystal orientation and which produces controllable vertical profiles in the crystal.
Recent developments in micromechanics have successfully led to the fabrication of microactuators utilizing processes which have involved either bulk or surface micromachining. The most popular surface micromachining process has used polysilicon as the structural layer in which the mechanical structures are formed. For such a polysilicon process, a sacrificial layer is deposited on a silicon substrate prior to the deposition of the polysilicon layer. The mechanical structures are defined in the polysilicon, and then the sacrificial layer is etched partially or completely down to the silicon substrate to free the polysilicon movable mechanical structures. Such polysilicon technology is not easily scaled for the formation of submicron, high aspect-ratio mechanical structures, because it is difficult to deposit fine-grain polysilicon films to the required thickness.
Some bulk micromachining processes can yield mechanical single-crystal silicon structures using wet chemical etchants such as EDP, KOH, and hydrazine to undercut single-crystal silicon structures from a silicon wafer. However, such processes are dependent on crystal orientation within the silicon wafer, and for this and other reasons the type, shape and size of the structures that can be fabricated with the wet chemical etch techniques are severely limited.
The use of single-crystal materials for mechanical structures can be beneficial, since these materials have fewer defects, no grain boundaries and therefore can be scaled to submicron dimensions while retaining their structural and mechanical properties. Also, the use of single-crystal materials, particularly single-crystal Silicon (Si), Silicon Germanium (SiGe), Indium Phosphene (InP) and Gallium Arsenide (GaAs), to produce mechanical sensors and actuators can facilitate and optimize electronic and photonic system integration. Single crystal gallium arsenide (SC-GaAs) in particular is an attractive material for micromechanics because of its optoelectronics properties. Furthermore, the use of SC-GaAs to produce mechanical sensors and actuators permits the integration of mechanical, electronic and photonic devices, However, the brittleness and the thermal expansion mismatch between dielectric thin films and SC-GaAs complicate the fabrication of three-dimensional GaAs mechanical structures.
In copending U.S. application Ser. No. 07/821,944, of Noel C. MacDonald and Zuoying L. Zhang, filed Jan. 16, 1992, U.S. Pat. No. 5,198,390 and entitled "RIE Process for Fabricating Silicon Electromechanical Structures", a reactive ion etching process is disclosed for the fabrication of submicron, single-crystal silicon movable micromechanical structures. That process, known as a single-crystal reactive etch and metallization process (SCREAM), provides a significant advantage in the manufacture of silicon structures since no thick film deposition process is required. Instead, reactive ion etching (RIE) processes are used to fabricate released SCS structures with lateral feature sizes down to 250 nm and with arbitrary structure orientation on an SCS wafer. The SCREAM process includes options to make integrated, side-drive capacitor actuators, the capacitor actuators being formed by means of a compatible, high step-coverage metallization process using aluminum sputter deposition and isotropic aluminum dry etching. The metallization process is used to form side drive electrodes and complements the silicon RIE processes used to form these structures.
In general, the SCREAM process of that application defines mechanical structures with one mask, with the structures being etched from a substrate. In one embodiment of the invention, the starting substrate is a silicon wafer on which a layer of silicon dioxide approximately 400 nm thick is thermally grown, this material then being available for use as an etch mask. The pattern to produce free standing SCS structures is created using photolithography in a photoresist spun on the silicon dioxide layer, and this photoresist pattern is transferred to the silicon dioxide by a reactive ion etching step. The photoresist is then stripped and the silicon dioxide pattern is transferred to the silicon substrate using a second RIE process, forming trenches and islands in accordance with the desired structural features in the silicon. As an option, contact windows may be opened in the silicon dioxide to allow electrical contact to both the silicon substrate and the movable silicon structures, and thereafter a 400 nm layer of aluminum is conformally deposited using DC magnetron sputter deposition. This aluminum makes electrical contact with the silicon substrate and with the movable silicon structures through the contact windows, while the remainder is deposited on the silicon dioxide layer, A photoresist is used to re-fill the etched silicon trenches following this aluminum sputter deposition and thereafter aluminum side electrode patterns are produced in the photoresist through the use of photolithography. This pattern is then transferred to the aluminum layer by means of an isotropic RIE, with the photolithography and RIE steps producing smooth edges on the aluminum pattern.
After the aluminum electrodes are patterned, an etching step removes the silicon dioxide on the bottoms of the trenches, while leaving the silicon dioxide on the top and side walls of the structures previously defined in the substrate. The silicon mechanical structures are released from the silicon substrate using a further RIE process, with the top and side walls of the structures being protected by the silicon dioxide mask during undercutting. Finally, the resist which was used for the aluminum patterning is stripped from the structure by a suitable plasma etch.
The foregoing process as described in the aforesaid copending patent application, can be used to fabricate complex polygonal shapes, including triangular and rectangular structures, as well as curved structures such as circles, ellipses and parabolas in single-crystal silicon. Such structures can include integrated, high aspect-ratio and conformable capacitor actuators, as required. Thus, it is possible to form suspended SCS structures with complex shapes from a silicon wafer through a simplified RIE process.
The foregoing process relies upon the formation of a silicon dioxide layer on the single-crystal silicon material; however, it is often desirable to utilize materials in addition to silicon, such as GaAs, SiGe and InP, as well as superlattices in such structures, as noted above, but such other materials do not generate an oxide layer in the manner of silicon. Accordingly, a different process is required to produce released micromechanical structures in materials other than silicon.